-Firstly, is there a high risk that I'll have to pay a tonne of customs importing the HA-S680s from Japan to the UK?

I've been planning on buying the CALs for a few months now, but I also realized I can get the JVCs for $88 too, which would you recommend, or perhaps others?

I listen to mainly hip-hop, my current headphones are too "warm" sounding. The bass is bloated and sloppy, leaking into the mid-tones of the song, the overall soundstage of the headphone is far too laid back (they're cheapish headphones, just Sony MDR 570LPs).

What would you recommend for around $85? I'd like an aggressive and very CLEAR sound for hip-hop, definitely not warm and sloppy.

No headphones really spring to mind. The cal is a warm sounding one. You are probs going to pay for importing those jvcs. Try search for budget headphones, I remember there being at least one very good thread on it.